Field of the Invention
The present invention relates generally to prosthetic devices for implantation between adjacent vertebrae to treat patients with ruptured or severely degenerated intervertebral disks, and more particularly to a more easily implantable two-piece intervertebral disk prosthesis for insertion intermediate two adjacent vertebrae to maintain the aforesaid vertebrae in a properly spaced-apart and stable configuration which will be resistive to normal compressive forces exerted on the spine while bony integration is occurring to fuse the aforesaid vertebrae together.
The main structural support of the human skeleton is the spinal column, a bony column that consists of a plurality of vertebrae which are interlinked by flexible joints, spaced apart by gelatinous intervertebral disks of fibrocartilage, and held together by ligaments. Each vertebra has a roughly cylindrical body, with wing-like projections, and a bony arch. The arches, which are positioned next to one another, create a tunnel-like space which houses the spinal cord. The cylindrical bodies of the vertebrae are spaced apart by intervertebral disks, and bear most of the compressive load of the spinal column (approximately 80 percent of the total load).
One of the primary causes of back of neck and back pain in patients is when one of these intervertebral disks becomes degenerated or ruptured, no longer spacing the adjacent vertebrae on opposite sides of the intervertebral disk properly apart. When an intervertebral disk becomes degenerated or ruptured to the point where it no longer functions properly, a diskectomy, i.e. removal of the degenerated or ruptured disk from its location between adjacent vertebrae, is indicated, and this treatment has become the classic treatment for the dysfunctional disk.
Following the diskectomy, the two adjacent vertebrae on opposite sides of the removed disk are typically fused together in a process which has as its objective surgically reconstructing the space intermediate the two vertebrae. Two different techniques may be utilized to perform this surgical reconstruction of the intervertebral space. These first of these two techniques is the placement of an autograft, i.e. a bone graft, intermediate the two vertebrae, and the second technique is the installation of an intervertebral prosthesis intermediate the two vertebrae.
The use of autografts has been well established, and the procedure places the autograft between the superior endplate of one vertebra and the inferior endplate of the second vertebra. The autograft bears weight between the two vertebrae, and ultimately fusion occurs between the two vertebrae and the autograft. The material for the autograft may be either material excised from the patient's own fibula or pelvis, and shaped by the surgeon to fit in the intervertebral space. Alternately, an allograft, which is bone which typically has been harvested from a deceased donor, may be specially prepared and sterilized, and then shaped by the surgeon to fit properly.
The alternative to the use of autografts is the use of prosthetic intervertebral implants, which are typically made of titanium and, in some cases, graphite. A wide variety of intervertebral implants have been proposed, most of which require complex installation procedures. These prosthetic devices include wedges, solid cylinders, hollow cylinders, and complex multi-component devices having a number of specially machined parts. Many of these prosthetic devices require special preparation of the two vertebrae between which the intervertebral disk was removed, with such preparation including precision drilling or shaving of the vertebrae to create various apertures, grooves, sockets, etc.
Placement of the prosthetic devices into the intervertebral area may also be a difficult and time consuming process, particularly if the surgery is performed from the posterior side of the spine. This is due to the simple fact that the endplates of the vertebrae are located on the anterior-facing side of the spine, with the spinal cord located between the back (from which the surgical approach is typically made) and the endplates of the vertebrae.
Thus, it will be appreciated by those skilled in the art that if the intervertebral prosthesis is to be inserted from the posterior, it may be difficult to maneuver the prosthesis into place. While the cervical and lumbar regions of the spine may be reached from an anterior position, the thoracic region of the spine is almost always accessed from the posterior position. Thus, many of the intervertebral prostheses previously known are simply less than ideal since they are quite difficult to properly place.
In addition, many of the intervertebral prostheses known in the art also require a great deal of vertebrae preparation. Some of these prostheses require that the facing endplates of the vertebrae be drilled and/or milled to precise shapes, while others require angles of approach which are quite difficult to attain, even when the surgeon is highly skilled in the procedure. Many of these intervertebral prostheses are far from ideal in construction and reliability, and a number of them simply fail over time since they do not adequately facilitate bone ingrowth.
It is accordingly the primary objective of the present invention that it provide an improved intervertebral disk prosthesis which may be used to replace an excised intervertebral disk and restore proper spacing between adjacent vertebrae which have had the intervertebral disk removed. It is an objective of the intervertebral disk prosthesis of the present invention that be of a design and physical configuration which may be easily installed in place intermediate the endplates of two adjacent vertebrae via a posterior surgical approach. It is a related objective of the intervertebral disk prosthesis of the present invention that it not require extensive drilling or other precision shaping of the endplates of the adjacent vertebrae prior to installation. It is also an objective of the intervertebral disk of the present invention that the implant procedure not require the use of complex tools to install and position the intervertebral disk prosthesis intermediate the two vertebrae.
It is an further objective of the intervertebral disk prosthesis of the present invention that it be implantable in a surgical procedure reducing both the trauma to the patient and the time for the surgeon to implant the device. It is also an objective of the intervertebral disk prosthesis of the present invention that it be highly resistive to dislodgement once it is initially installed intermediate two vertebrae. It is yet another objective of the intervertebral disk prosthesis of the present invention that it promote prompt and permanent ingrowth of bone material intermediate the two adjacent vertebrae to facilitate permanent fusion of the aforesaid vertebrae. Still further objectives of the intervertebral disk prosthesis of the present invention are that it be made of biocompatible material compatible with long term implant in the human body, and that it be available in different sizes and configurations to fit a wide variety of patients and different disk locations in the spine.
The intervertebral disk of the present invention must be of a construction which is both durable and long lasting, and it must require no maintenance once it is implanted. In order to enhance the market appeal of the intervertebral disk prosthesis of the present invention, it should also be of a simple mechanical design and relatively inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives of the intervertebral disk prosthesis of the present invention be achieved without incurring any substantial relative disadvantage.